Abstract
Due to substrate layers with different substrate configurations, extensive green roofs (EGRs) exhibit different rainfall runoff retention and pollution interception effects. In the rainfall runoff scouring process, nutrient leaching often occurs in the substrate layer, which becomes a pollution source for rainwater runoff. In this study, six EGR devices with different substrate layer configurations were fabricated. Then, the cumulative leaching quantity (CLQ) and total leaching rate (TLR) of NH4+, TN, and TP in the outflow of nine different depth simulated rainfall events under local rainfall characteristics were evaluated and recorded. Furthermore, the impact of different substrate configurations on the pollution interception effects of EGRs for rainfall runoff was studied. Results show that a mixed adsorption substrate in the EGR substrate layer has a more significant rainfall runoff pollution interception capacity than a single adsorption substrate. PVL and PVGL, as EGRs with layered configuration substrate layers, exhibited good NH4+-N interception capacity. The CLQ and TLR of NH4+-N for PVL and PVGL were - 114.613mg and - 63.43%, - 121.364mg, and - 67.16%, respectively. Further, the addition of biochar as a modifier significantly slowed down the substrate layer TP leaching effect and improved the interception effect of NH4+-N and TN. Moreover, although polyacrylamide addition in the substrate layer aggravated the nitrogen leaching phenomenon in the EGRs' outflow, but the granular structure substrate layer constructed by it exhibited a significantly inhibited TP leaching effect.
Highlights
In recent years, the urbanization process in China has accelerated, and the hardening rates of roads and roofs have sharply increased
According to the short-duration and high-intensity rainfall characteristics in Nanchang and considering the study by Dimitar et al (2018), when the interval of the rainfall events is greater than two days, the rainfall runoff retention capacity of the Extensive green roofs (GRs) (EGRs) substrate layer will not be affected by the last rainfall event
Based on the water quality measurement data in the experiment, this study evaluates the nutrient leaching control ability of the EGR substrate layer through two indicators: cumulative leaching quantity (CLQ) and total leaching rate (TLR)
Summary
The urbanization process in China has accelerated, and the hardening rates of roads and roofs have sharply increased. Extreme rainstorms have frequently occurred, causing a large number of pollutants to be washed away by runoff into receiving water This has led to urban waterlogging and non-point source pollution phenomena. Numerous researchers have reached a consensus and have verified through practical engineering applications and experimental studies that the EGR substrate layer can effectively delay outflow, retain runoff and reduce peak flow during the rainfall process (Vijayaraghavan et al.2019; Gong et al 2018). Due to the different substrate types, ratios and configurations, the pollution interception effectiveness of EGRs for rainfall runoff is quite different and often appears as a pollution source for rainfall runoff (Karczmarczyk et al 2018; Chai et al 2018; Zhang et al 2018) This is because of the leaching of the nutrients in the substrate layer during the runoff scouring process. This study provides a reliable basis and reference for constructing of EGRs and EGR substrate layers in practical engineering applications and experimental research
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